The invention pertains to an excavating tool such as, for example, a rotary drill bit, including the cutting insert therefor, and a method of drilling using the rotary drill bit, wherein the bit is useful for drilling through various earth strata. More specifically, the invention pertains to a roof drill bit, including the cutting insert therefor, and a method for using the roof drill bit, wherein the bit is useful for drilling bore holes in an underground mine.
The expansion of an underground mine, such as for example, a coal mine, requires digging a tunnel. Initially, this tunnel has an unsupported roof. Because the roof is not supported, there is an increased chance for a mine cave which, of course, adds to the hazards of underground coal mining. Furthermore, an unsupported roof is susceptible to rock and debris falling from the roof. Falling rock and debris can injure workers as well as create hazardous clutter on the floor of the tunnel.
In order to support and stabilize the roof in an underground tunnel, bore holes are drilled in the roof, i.e., earth strata. The apparatus used to drill these holes comprises a drill with a long shaft, i.e., drill steel, attached to the drill. A roof bit is detachably mounted to the drill steel at the distal end thereof. The roof bit is then pressed against the roof, and the drilling apparatus operated so as to drill a bore hole in the roof. The bore holes extend between about two feet and about twenty feet into the roof depending upon the particular situation. The typical rate of rotation is between about 250 to about 600 rpm, and the typical thrust is between about 100 to about 10,000 pounds for a time sufficient to drill the desired hole in the earth strata.
These bore holes are filled with resin and roof bolts are fixed within the bore holes. Roof support members, such as roof panels, are them attached to the roof bolts. The end result is a roof which is supported, and hence, is of much greater stability than the unsupported roof. This reduces the hazards associated with underground mining. The roof bolting process is considered to be an essential underground mining activity.
Roof bolting accounts for the largest number of lost time injuries in underground mining. During the roof bolting process, the roof is unsupported so that it does not have optimum stability. Furthermore, the roof bolting process exerts stresses on the roof so as to further increase the safety hazards during the roof bolting process. Thus, a decrease in the overall time necessary to bore holes reduces the time it takes to complete the roof bolting process. This is desirable since it contributes to the overall speed, efficiency and safety of the roof bolting process.
On occasion in the past, the roof bit would "stick", i.e., become bound, in the bore hole. When this occurs, it is necessary to halt the drilling, and then exert forces, and possibly impacts, to the roof bit to extract it from the bore hole. Sticking of the roof bit in the bore hole consumes additional time to remove it from the bore hole, and thereby lengthens the time necessary to complete the overall roof bolting process. Furthermore there is a chance that the cutting insert can be broken in an attempt to remove the roof bit from the bore hole. It thus would be desirable to provide a roof bit that does not have the tendency to stick in the bore hole during the drilling of bore holes.
The typical cutting insert for a roof bit is made from a cemented tungsten carbide, which is comprised of cobalt and WC. The cemented tungsten carbide degrades upon continuous exposure to high temperatures. In a typical roof bolting operation, the cemented tungsten carbide insert is subjected to high temperatures during the drilling of the bore holes. Exposure to these high temperatures tends to increase the wear rate of the cutting edge of the cutting insert which results in a slow down of the drilling operation. Upon the degradation of the cutting edge of the cutting insert during the drilling, the roof bit must be removed and replaced with another roof bit. It would be therefore desirable to provide a roof bit wherein the cemented tungsten carbide cutting insert thereof is structured so that the heat generated during drilling is reduced from earlier levels, thereby reducing the tendency of the cutting edge of the cemented carbide cutting insert to degrade, and hence, became dull so as to slow down the drilling operation.
Roof bits have also used cutting inserts that typically include a cemented tungsten carbide backing with a layer of polycrystalline diamond thereon. The polycrystalline diamond is heat sensitive so that the tendency of a bit to not generate as much frictional heat as in the past would be a desirable feature for roof bits that use a cutting insert that includes a layer of polycrystalline diamond.
Upon the completion of drilling in a bore hole, the drilling apparatus is oftentimes spun backwards so as to remove the roof bit from the bore hole. On occasion, the cemented tungsten carbide insert will become chipped upon the backwards rotation of the roof bit. Chipping of the cemented tungsten carbide cutting insert will sometimes necessitate replacement of the roof bit. Thus, it would be desirable to provide a roof bit with a cemented tungsten carbide insert that is structured so as to reduce the tendency to chip when the roof bit is spun backwards.
In the manufacture of the roof bit, one of the more expensive components thereof is the cemented tungsten carbide insert. Thus, it would be desirable to provide an improved roof bit wherein the amount of cemented tungsten carbide necessary to manufacture the cutting insert thereof is less than in earlier roof bits.